This non-provisional application is based upon a provisional application filed on Jun. 29, 2001 for which priority is claimed.
The present invention relates to handling of,nuclear fuel assemblies. More specifically, the present invention provides a structural reinforcement for a nuclear fuel assembly and method for installation and use of such a reinforcement.
Handling of heavy objects is a necessary critical activity performed for safe and continuous operation of nuclear power plants. Lifting of nuclear fuel assemblies is often strictly regulated with necessary precautions being undertaken prior to actual load lift. Typical precautions include, for example, ascertaining the weight of the assembly to be lifted, determination of a path that the assembly take during the lift, identification of critical and/or safety sensitive nuclear related components which may be jeopardized during the lift, and evaluation of potential damage in the event of a load drop.
The fuel assemblies provided to a nuclear reactor are composed of a plurality of rods which are supported by a lower nozzle or end fitting. A top nozzle or end fitting is located above the plurality of rods. Interspersed in the plurality of fuel rods are guide thimbles which allow insertion of rod absorber material into and out of the plurality of fuel rods to help control the nuclear reaction. The guide thimbles project between the upper and lower nozzles or end fittings. Between the upper and lower nozzles or end fittings a plurality of spacer grids are positioned at intervals to provide lateral support and prevent destructive side to side movement of individual fuel rods. The top nozzle or end fitting is positioned at the top of the fuel assembly and connects to the guide thimbles via sleeves to allow the load of the entire fuel assembly to be carried from the lower nozzle, up the guide thimbles to the upper end fitting. An instrument tube is located in the interior of the fuel assembly wherein the tube allows instrumentation to be inserted into the assembly during operation or maintenance activities. The fuel assembly top nozzle and bottom nozzle are configured to aid in channeling coolant flow through the assembly during operation. In this configuration, the weight of the fuel rods is born by the guide thimbles and not by the fuel rods when a fuel assembly is lifted by the upper end fitting.
Certain areas in a nuclear power plant or similar facility, are more safety sensitive and require extremely thorough procedures to ensure continued health and safety of the public at large. Such safety sensitive areas include the nuclear reactor itself and the fuel pool. The presence of potentially large amounts of radioactive material in these areas, as well as vital cooling systems for the plant, requires the utmost care when performing lifting in these areas. The required safety in these areas necessitates several additional checks before initiating such evolutions. These checks usually involve, for example, inspecting the structural components of the polar crane or a fuel handling crane. In addition to inspecting of the crane, the actual load lifted (i.e. the fuel assembly itself which will be carried by the crane is inspected and evaluated.
Nuclear plant operating experience has identified that guide thimbles of fuel assemblies, in some cases, exhibit stress corrosion cracking or other structural defects. Specific experience has shown that stress corrosion cracking of the upper guide thimble sleeves, which attach the upper end fitting to-the guide thimbles, can be a significant problem. Numerous documented instances of degradation have been ascertained in the upper end fitting to guide thimble connection, resulting in a potentially dropped fuel assembly accident if a defective fuel assembly is lifted. Current practices in the industry require costly and time consuming alterations to damaged fuel assemblies to allow lifting to occur.
The design of the connection between the upper end fitting and the remainder of the fuel assembly only provides a single load path where the weight of the fuel assembly is supported (i.e. using the guide thimble sleeves). A failure of this single load path, therefore, will result in the fuel assembly being dropped.
There is a need, therefore, to provide a repair to fuel assemblies which exhibit stress corrosion cracking problems or other similar load path defects without incurring costly and time consuming alterations.
There is a further need to provide a redundant load path for a fuel assembly which will be lifted during maintenance activities.
It is an object of the present invention to provide a fuel assembly structural reinforcement to prevent accidental fuel assembly drop when the fuel assembly exhibits stress corrosion cracking or has another defect in the structural load path.
It is also an object of the present invention to provide a method to repair a damaged fuel assembly to allow the fuel assembly to be safely lifted.
It is also an object of the present invention to provide a redundant load path for a nuclear fuel assembly.
The invention comprises an apparatus to repair a fuel assembly to accomplish a load lift comprising a main body, an upper section, a connection configured between the main body and the upper section and a flower section connected to the main body wherein the lower section is configured to form a holding body. The invention also comprises an inner adjusting body with a top and a bottom wherein a mandrel is connected to the bottom, the mandrel configured to actuate the holding body upon actuation of the inner adjusting body, the inner adjusting body positioned inside the upper section, the main body and the lower section and an actuator connected to the upper section, the actuator configured to actuate the inner adjusting body.
The invention also provides a method for providing a fuel assembly structural reinforcement. The method comprises repairing a fuel assembly to accomplish a load lift including machining a hole through a grillage of a top nozzle of the fuel assembly to access an instrument tube, inserting a reinforcement into the hole to a bottom nozzle of the fuel assembly, and actuating the reinforcement to provide a secondary load path.